This invention relates in general to attenuator circuits employing a field effect transistor (FET) and more specifically to a circuit employing an impedance and a FET, where the FET provides a conductance as a desired predetermined function of a control signal, in order to provide a standardized gain control or frequency response characteristic.
Three terminal field effect transistors have been used frequently to provide variable conductances where the conductances between the source and drain are controlled by control signals applied to the gates of the FETs. The drain-source conductances of most FETs vary linearly with the control voltage applied across the source and gate so that incremental changes in the control voltage will cause proportional incremental changes in their conductances. The variation of the drain-source conductance of a FET as a function of the control signal is known as the conductance control characteristic of the FET.
Field effect transistors used as variable conductances have been employed in signal attenuators. For example, they have been used in limiters for noise reduction systems where compressors and expanders require controlled limiting of the amplitude of a signal. In U.S. Pat. No. 4,490,691, a field effect transistor is used in conjunction with a capacitor to form a high pass filter with a sliding band type high frequency shelf response whose corner frequency varies as a function of the control signal to the gate of the transistor. In this particular high pass filter circuit the drain-source path of the FET shunts the signal to be limited to ground. In other attenuator or limiter circuits, the drain-source conductance of the FET may be placed in the series path of the signal instead of in a shunt path. A FET can be used in conjunction with an inductor to form a sliding band circuit with a low frequency band pass response operating downwardly. A FET may also be used in conjunction with a resistor and another filter to form a fixed band attenuator circuit whose frequency range does not change, in contrast to the sliding band filter circuits. In each of the above types of attenuators, the characteristics of the attenuator are varied by varying the conductance of the drain-source path of the FET.
In many attenuator, limiter and filter circuits it is important for the circuits to have precise and reproducible characteristics. For noise reduction systems employing complementary compressors and expanders, for example, it is important for the signal limiters and filters in a group of matching compressors (or expanders) to have similar characteristics. This enables the original uncompressed signal to be recovered by any expander complementary to the group of compressors from a compressed signal compressed by a compressor in the group, and enables the use of any matching compressor to produce the compressed signal.
In order that the signal attenuators, limiters or sliding band circuits in the compressors (or expanders) have similar signal attenuating characteristics, it is desirable for the FETs in such circuits to have similar conductance control characteristics. In other words, the conductances of the FETs in the different limiter circuits in the compressors (or expanders) are preferably the same in response to the same control signals. It is well known that the conductance control characteristics of FETs vary widely, so that different FETs may have very different conductances even though the same control signal is applied to their gates. It is therefore desirable to select or fabricate FETs or otherwise provide FET circuits which will have the same conductance in response to the same control signal.
Until now, the favorite method for providing FETs having the same conductance control characteristic is to use special manufacturing processes which may be expensive and difficult. Simpler and less expensive methods are thus desirable. In a method employed by Dolby Laboratories of San Francisco, Calif. to provide reproducible FET conductance control characteristics, the FETs used are selected so that a control signal fed to their gates will produce essentially the same conductances. Then, offset voltages are applied where necessary to compensate for the difference in pinch-off voltages of the selected FETs.
In U.S. Pat. No. 3,818,244 and 3,737,678 to Dolby et al, reproducibility of FET conductance control characteristics is improved by employing two FETs with pinch-off voltages selected to be identical to provide the attenuation required through two stages. The first FET provides the first few dB of attenuation, with most of the remaining required attenuation provided by the second FET, the two FETs having different thresholds of control voltage at which they start to conduct. A high degree of repeatability is achieved even at high values of attenuation because the first FET is fully conducting after the first few dB of attenuation, giving an accurate value of first stage attenuation.
In U.S. Pat. No. 2,035,263, Cushman et al. propose to use two identical variable resistance devices controlled in the same manner by the same control signal to provide a compression or expansion ratio of 2:1. In other words, for devices having a wide variety of conductance control characteristics, a precise and reproducible compression or expansion ratio of 2:1 can be achieved so long as the two variable resistance devices used in the compressor or expander are identical and identically controlled. In such manner matching companders can be produced even though the characteristics of variable resistance devices in one compander do not match those of devices in a different compander. Any number of variable resistance devices may be employed to provide different fractional compression and expansion ratios.
In the article "Compander Increases Dynamic Range" by Wermuth, db Magazine, June 1976, the use of three identical FETs (FIG. 8) is proposed in a scheme similar to that of Cushman et al's to achieve a compression ratio of 2:3 or by 33% on a logarithmic scale; the scheme enables the characteristic to be reproducible. In U.S. Pat. No. 3,969,680 Wermuth proposes the use of two identical FETs in another similar scheme to provide constant slope 2:1 compression and expansion.
None of the above described conventional methods is entirely satisfactory. In some methods the FETs used must be selected from a limited group. For companders using identical devices, special manufacturing may again be required to produce these devices. Furthermore, the transient response of such companders may deviate significantly from the constant ratio in a manner dependent on the actual conductance control characteristics of the identical loss devices.